The invention relates to polyether-modified, water-dispersible polyisocyanate mixtures, a process for their production and their use as initial component in the production of polyurethane plastics, in particular as crosslinker for water-soluble or water-dispersible lacquer binding agents or binding-agent components with groups capable of reacting with isocyanate groups.
Against the background of increasing environmental awareness, water-dispersible polyisocyanates have gained in importance in recent years for various fields of application.
Water-dispersible polyisocyanates play a special role nowadays as crosslinker components for water-dilutable two-component polyurethane (2C PUR) lacquers. In combination with aqueous polyol dispersions they permit the formulation of solvent-free lacquer systems that cure already at room temperature to form high-quality coatings which are in no way inferior to conventional lacquers in respect of their resistance to solvents and chemicals or their mechanical load-bearing capacity (cf e.g. EP-A-0 358 979, EP-A-0 469 389, EP-A-0 496 210, EP-A-0 542 105, EP-A-0 543 228, EP-A-0 562 282, EP-A-0 562 436, EP-A-0 583 728, DE-A-4 129 951, DE-A-4 226 242, DE-A-4 226 243 or DE-A-4 226 270).
In addition, water-dispersible polyisocyanate preparations are also important as additives for aqueous dispersion adhesives. With their aid it is possible, for example, for the resistance to heat and the resistance to water of adhesive bonds of different materials to be improved considerably (cf. e.g. EP-A-0 061 628 and EP-A-0 206 059).
Water-dispersible polyisocyanates are used, in addition, as crosslinker components for aqueous dispersions in the finishing of textiles (EP-A-0 560 161 or WO 95/30045) or in formaldehyde-free printing inks for textiles (EP-A-0 571 867 or DE-A-19 533 218) and furthermore are also suitable, for example, as auxiliary agents for the wet strengthening of paper (EP-A-0 564 912, EP-0 582 166, EP-A-0 707 113, WO 96/20309 and WO 97/04169).
In practice, non-ionic polyisocyanates that have been modified with the aid of polyethers to be hydrophilic have gained acceptance for these fields of application. The production of such water-dispersible polyisocyanates is described in a number of publications.
According to the teaching of DE-A-2 415 435, urethanes that are formed from organic, in particular aromatic, polyisocyanates and polyethylene glycol monoalkyl ethers comprising at least 5 ethylene-oxide units, for example, constitute surface-active substances which according to GB-A-1 444 933 and DE-A-2 908 844 enable the production of stable aqueous emulsions of aromatic polyisocyanates.
As a result of conversion with alkylene oxide polyethers, aromatic polyisocyanates that have been modified to be hydrophilic are also known from EP-A-0 061 628 and EP-A-0 095 594. In the form of aqueous emulsions these products are used, in particular, in the field of adhesives.
Water-dispersible preparations of (cyclo)aliphatic polyisocyanates are the subject of EP-A-0 206 059. By way of emulsifiers they contain conversion products that are formed from polyisocyanates with monovalent or polyvalent polyalkylene oxide alcohols, consisting of at least one polyether chain comprising at least 10 ethylene-oxide units, and serve likewise as additives for aqueous adhesives.
EP-0 516 277 describes the hydrophilisation of special polyisocyanates comprising tertiary-bound isocyanate groups by conversion with monovalent polyalkylene oxide polyethers and the use of these products as crosslinker component for aqueous coating agents.
For high-grade, light-fast lacquer applications, in particular the polyisocyanate mixtures that are described in EP-A-0 540 985 and U.S. Pat. No. 5,200,489 and that are obtainable by urethanisation of aliphatic and/or cycloaliphatic lacquer polyisocyanates with short-chain polyethylene oxide polyether alcohols comprising, on statistical average, fewer than 10 ethylene-oxide units have proved their worth.
The water-dispersible polyisocyanates known from EP-A-0 645 410 and EP-A-0 680 983 as crosslinkers for aqueous lacquers for wood and furniture, based on 2,4(6)-diisocyanatotoluene TDI) or mixtures of TDI and 1,6-diisocyanatohexane (HDI), also contain, by way of hydrophilic constituents, urethanes that are formed from polyisocyanate and monofunctional polyethylene oxide polyether alcohols.
In addition to these purely non-ionic, hydrophilised polyisocyanates containing polyether urethanes, polyether-modified water-dispersible polyisocyanates have also been described that comprise in addition, with a view to improving their emulsifiability or with a view to achieving special effects, also ionic groups, for example sulfonate groups (cf. e.g. EP-A-0 703 255) or amino or ammonium groups (cf. e.g. EP-A-0 582 166 and EP-A-0 707 113). Such ionically/non-ionically modified polyisocyanates are, as a rule, less suitable for lacquer applications. They preferably find application in the environmentally friendly finishing of textiles or as wet-strength agents for paper.
Despite their widespread market acceptance for an extremely wide range of applications, the water-dispersible polyisocyanates of the state of the art that have been modified with polyether urethane have a number of basic disadvantages.
Thus water-dispersible polyisocyanates that have been produced using high-molecular-weight polyether alcohols, for example in the case of pure polyethylene oxide polyethers starting from an average molecular weight of about 700, can, by reason of a very high maximum viscosity to be overcome during dispersal, often only be worked into aqueous media homogeneously by applying considerable shear forces (e.g. high-speed stirrers). In addition, such products, particularly in the case of high emulsifier contents such as are required in order to obtain particularly fine-particle dispersions that are stable as regards sedimentation, frequently have a tendency to crystallise.
By using shorter polyether chains, on the other hand, it is possible for water-dispersible polyisocyanates to be obtained that can be stirred into water very easily by hand so as to form stable dispersions and that, even in the case of high degrees of hydophilisation, i.e. with high contents of ethylene-oxide units, show no tendency towards crystallisation whatever. However, by reason of the relatively low molecular weight of the polyalkylene oxide polyethers that are employed for the purpose of modification, with increasing degree of hydrophilisation both the content of isocyanate groups and the average isocyanate functionality steadily diminish. In practice, however, for the majority of the aforementioned fields of applicationxe2x80x94for example as crosslinker components for lacquers and coatingsxe2x80x94highly hydrophilised polyisocyanates to be emulsified in the form of especially fine particles and having high NCO contents and as high a functionality as possible are precisely what are desired.
DE-A-19 822 891 describes, for the first time, a process for producing water-dispersible polyisocyanate mixtures that are not burdened with the disadvantages stated above. In this case, low-monomer polyisocyanates consisting of at least two diisocyanate molecules are converted with monofunctional polyethylene oxide polyether alcohols under allophanation conditions in such a way that at least 60 mol. %, preferably at least 80 mol. %, in particularly preferred manner at least 90 mol. %, of the urethane groups that are formed primarily by NCO/OH reaction continue reacting to form allophanate groups. The resulting polyisocyanate mixtures, which have a degree of allophanation of at least 60 %, can be stirred considerably more easily and in the form of finer particles into aqueous systems, even with clearly lower degrees of hydrophilisation, than water-dispersible polyisocyanates that have been produced using polyether alcohols of like structure in which the polyether chains are linked to the polyisocyanate via urethane bonds. In comparison with the polyisocyanate mixtures known previously, containing polyether chains and with the same or even better dispersibility in water, they are distinguished by a higher content of isocyanate groups and a higher functionality.
As has now been found, polyisocyanate mixtures that have likewise been produced under allophanation conditions from low-monomer polyisocyanates consisting of at least two diisocyanate molecules and from monofunctional polyethylene oxide polyether alcohols exhibit, in comparison with the polyether allophanates described in DE-A-19 822 891, once again a clearly improved dispersibility in water if the allophanation reaction is terminated before a degree of allophanation of 60% is attained. In this way it is possible for water-dispersible polyisocyanate mixtures to be produced that exhibit, in addition to a dispersibility that is improved once again, at the same time the advantage of a higher NCO content and that are suitable for use in all the, aforementioned fields of application of hydrophilic polyisocyanates, in particular as initial components for the production of polyurethane plastics and, above all, as crosslinkers for aqueous binding agents or binding-agent components in coating systems.
Although in some publications having as their subject-matter the production of polyisocyanates containing allophanate groups, for example in EP-A-0 000 194, EP-A-0 303 150, EP-A-0 682 012, U.S. Pat. No. 5,380,792 or U.S. Pat. No. 5,086,175, the conventional polyether alcohols are also named as possible alcoholic initial compounds for the production of such products and, in addition to these, in EP-A-0 000 194, EP-A-0 303 150 and EP-A-0 682 012, within long lists of suitable initial isocyanates, polyisocyanates with a functionality  greater than 2, such as, e.g., trimerisation products of HDI or 1-isocyanato-3,3,5-trimethyl-5-isocyanatomethylcyclohexane (IPDI), are also mentioned in all-inclusive manner, a person skilled in the art could not gather from any of the aforementioned publications any concrete indication at all that conversion products of low-monomer polyisocyanates with monofunctional polyethylene oxide polyether alcohols that are produced under allophanation conditions with degrees of allophanation from 20 to 59% can be stirred into water considerably more easily and in the form of finer particles so as to form stable emulsions than can polyisocyanate mixtures having the same gross composition that were produced in accordance with the known processes of the state of the art by urethanisation or in accordance with the process described in DE-A-19 822 891.
The present invention provides water-dispersible polyisocyanate mixtures based on aliphatic, cycloaliphatic, araliphatic and/or aromatic diisocyanates with
a) an average isocyanate functionality of at least 2.0,
b) a content of isocyanate groups (reckoned as NCO; molecular weight=42) from 5.0 to 25.0 wt. % and
c) a content of ethylene-oxide units bound within polyether chains (reckoned as C2H4O; molecular weight=44) from 2 to 50 wt. %, whereby the polyether chains contain, on statistical average, 5 to 35 ethylene-oxide units,
characterised in that the polyether chains are linked, in a proportion amounting to 20 to 59 mol. %, via allophanate groups to, in each case, two polyisocyanate molecules each synthesised from at least two diisocyanates.
The invention also provides a process for producing these water-dispersible polyisocyanate mixtures, which is characterised in that
A) a polyisocyanate component having an (average) NCO functionality from 2.0 to 5.0, a content of aliphatically, cycloaliphatically, araliphatically and/or aromatically bound isocyanate groups (reckoned as NCO; molecular weight=42) from 8.0 to 27.0 wt. % and a content of monomeric diisocyanates of less than 1 wt. % and
B) a monovalent polyalkylene oxide polyether alcohol comprising, on statistical average, from 5 to 35 ethylene-oxide units
are caused to react with one another, maintaining an NCO/OH equivalent ratio from 6:1 to 400:1, in such a way that 20 to 59% of the urethane groups that are formed primarily by NCO/OH reaction continue reacting to form allophanate groups, whereby in other respects the type and the quantitative ratios of the stated initial compounds are chosen in such a way that the resulting conversion products conform to the conditions stated above under a) to c).
The invention also provides the use of these polyisocyanate mixtures as initial components in the production of polyurethane plastics.
Finally, the invention also provides the use of these polyisocyanate mixtures as crosslinkers for water-soluble or water-dispersible lacquer binding agents or lacquer binding-agent components in the production of coatings by using aqueous coating agents based on such binding agents or binding-agent components.